British Pat. No. 792,598 to Ranby and corresponding J. Electrochemical Soc., Vol. 104, No. 10 (October 1957)pp. 612-615, article entitled "Cerium-Activated Halophosphate Phosphors" by S. T. Henderson and P. W. Ranby disclose a luminescent material comprising a halophosphate of calcium and/or strontium, having an apatite structure activated with cerium or with cerium and manganese, containing sodium.